I. Field of the Invention
The present invention relates to a magnetic recording medium and a method of manufacturing the same and, more particularly, to a rigid disc having an improved lubricant layer on its uppermost surface and a method of manufacturing the same. II. Description of the Prior Art
A recording media applies rigid discs that are incorporated in equipment such as host computer. In recent times, demand has increased for recording media having a high-density memory capacity. In order to satisfy this demand, a continuous thin-film type recording medium having a high coercivity has been developed, which is rapidly becoming more popular than the particulate type medium. In a thin-film type recording medium, a ferromagnetic layer is continuously formed on a non-magnetic substrate by sputtering, deposition, wet plating, or the like. As a result, a thin-film type magnetic recording medium can be obtained, having an extremely high recording density as compared to a particulate type medium. As a material for the ferromagnetic layer, Co-Ni, Co-Cr, Co, Co-Re, Co-Ni-W, Co-Pt, Co-Ni-Pt, Co-Sm, and the like are used in the case of sputtering, and Co-P, Co-Ni-P, Co-Ni-Mn-P, Co-Re-Ni-P, and the like are used in the case of plating. A surface hardening layer (an Ni-P plating layer, and the like) or an underlayer (e.g., Ti, Ge, or the like for a Co-Cr perpendicular magnetization film, and Cr or the like for a Co-Ni longitudinal magnetization film) for improving the characteristics of the magnetic layer is formed between the substrate and the magnetic layer. A lubricant layer (SiO.sub.2, C, or the like) is formed on the magnetic layer, for protection against head contact and for lubrication. These layers are often formed by sputtering. This is because sputtering allows selection of a variety of materials, and the easy manufacture of a structure with good magnetic characteristics and anti-corrosion properties. In addition, sputtering imparts good surface properties to the resultant film and allows easy process control.
If in a disc of a thin-film type magnetic recording medium, a Co-Ni alloy is used as a magnetic layer, the thickness of the magnetic layer should fall within the range of 500 .ANG. to 2,000 .ANG., in order to obtain a sufficiently high reproduction output. However, if the magnetic layer is formed of only the Co-Ni alloy, it will have poor anti-corrosion properties, and hence, a 1,000-.ANG. thick non-magnetic layer (e.g., Cr) with good anti-corrosion properties must be formed on the magnetic layer, in order to compensate for this shortcoming. In addition, a lubricant layer must then be formed on the non-magnetic layer. However, taking the reproduction output level into consideration, the distance at which a head floats above a rotating disc must fall within the range of 2,000 .ANG. to 3,000 .ANG.. If the non-magnetic layer has a thickness of about 1,000 .ANG., the head should be separated from the magnetic layer accordingly. This also results in a decrease in reproduction output, on account of a spacing loss. Therefore, in order to reduce the spacing loss, a lubricant layer formed on a surface region of the recording medium must be as thin as possible.
In addition, the lubricant layer formed on the uppermost surface of the magnetic recording medium must possess satisfactory anti-shock properties, since, upon power-off, the head falls onto the disc and comes into slidable contact with the lubricant layer. The lubricant layer must also have a high surface smoothness, in order that a stable reproduction output can be obtained. For this purpose, the lubricant layer must be able to remain free from damage even after it has been subjected to twenty to thirty thousand head-falls in a repetitive head-fall resistance test, known as a Contact-Start-and-Stop (to be referred to as CSS hereinafter) test.
In a particulate type magnetic recording medium, a lubricating oil is applied, in an appropriate quantity, to the surface of the disc, so as to impregnate the three-dimensional pattern of the surface layer. The oil oozes to the surface, thus forming a lubricant layer on the disc. However, as is disclosed in Journal of Applied Physics 55(6), if, in a thin-film type magnetic recording medium a lubricating oil in liquid form is applied to a very flat surface formed by sputtering or plating, a so-called head-attraction phenomenon occurs when a magnetic head is brought into contact with the disc surface. When this phenomenon occurs, the disc surface and the head cannot easily be separated from each other. Moreover, the thin-film type magnetic recording medium has no gap for storing the lubricating oil on the surface layer, unlike the particular type magnetic recording medium. Therefore, since the lubricating oil becomes deficient in it's lubricating properties after a short period of time, the medium cannot withstand long-term use.
For the above reasons, in the thin-film type magnetic recording medium, use of liquid lubrication is judged inappropriate, and hence, solid-state lubrication is adopted. As is disclosed in U.S. Pat. No. 4,552,820, graphite having a layer structure is found to be suitable as a solid-state lubrication material for covering the upper-most surface of a recording medium.
As is known, molybdenum disulfide can be used as another solid-state lubirication material. However, it is difficult to form a molybdenum disulfide layer of such a quantity as is suitable as a lubricant layer for the recording medium. For this reason, molybdenum disulfide is not used in practice.
When a solid-state lubricant layer is formed of graphite having a layer structure, a method for sputtering a carbon target in a rare gas atmosphere such as Ar gas is normally employed.
Although carbon graphite having the layer structure has satisfactory bonding properties in relation to the underlayer, the lubricant layer itself breakes up when the slide motion has repeated a relatively small number of times. As a result, the underlayer is exposed and is no longer protected against wear. When discs are mass produced, the reproducibility of the lubricant layer also presents a problem. That is, some of the manufactured media contain graphite having an excellent layer structure and thus possess sufficient durability, but the other media contain graphite having a poor layer structure and hence have not sufficient durability. In the media of insufficient durability, a shearing stress generated during a CSS test fails to act in a direction parallel to the lubricant layer, and the layer is broken up. Thus, the product yield is reduced.